In a four-wheel running vehicle equipped with a working implement which is movable upwardly and downwardly, it is already known to feed a working fluid to the same hydraulic system for lifting the working implement and also to a hydraulic steering system by a single hydraulic pump, as disclosed, for example, in U.S. Pat. Nos. 4,213,511 and 4,043,419.
In the prior art mentioned above, a variable displacement hydraulic pump is used as the hydraulic pump, and the rotation of the steering handle rotatingly operates a hydraulic steering valve and a hand-operated pump to reduce the influence of pressure acting between the hydraulic steering system and the hydraulic lifting system. For stable steering during running, this arrangement requires a large flow rate of working fluid and therefore a pump of large capacity is required. This entails great horsepower consumption for the hydraulic systems and necessitating a drive engine of increased horsepower. The hydraulic systems must also be composed of piping, filter, valve and the like of increased capacity. For this reason, it has been impractical to incorporate a hydraulic steering system into four-wheel running vehicles of small size.
Further with the above arrangement, the handoperated pump, which is provided for metering, must be rotated for manual steering while the drive engine is stopped.
The present invention is useful also in such a case, permitting manual steering by a simple structure.